Magnetic refrigerator having thermal valve means



Nov. 24, 1959 MAGNETIC REFRIGERATOR HAVING THERMAL VALVE MEANS OriginalFiled Oct. 15, 1956 R. L. GARWIN 2 Sheets-Sheet 1 OERSTEDS FiGJ VOLTAGESOURCE FIG.2

TEMPERATURE K I l READ GATE 8 AMPLIF IER f l i L- I ae'fiLow TEMPERATURESTORAGE CELL INVEN TOR. RICHARD L. GARWIN R Qua EMA/f ATTORNEY Nov. 24,1959 R. L. GARWIN MAGNETIC REFRIGERATOR HAVING THERMAL VALVE MEANSOriginal Filed Oct. 15, 1956 2 Sheets-Sheet 2 f4, f2, '15,]4, '5, ONESECOND T0100 SEC WIDTH -1OO uSEC \ALVE PULSE K MI INVENTOR RIC HARD L.GARWIN gkawiza/ ATTORNEY United States Patent MAGNETIC REFRIGERATORHAVING THERMAL VALVE MEANS Richard L. Garwin, Scarsdale, N.Y., assignorto International Business Machines N.Y., a corporation of New YorkOriginal application October 15, 1956, Serial No. 615,814. Divided andthis application November 27, 1957, Serial No. 699,398

8 Claims. c1. 62-3) The present invention relates to superconductingelements for storing induced persistent currents and more particularly,to the use of induced persistent currents for controlling the thermalconductivity of other elements having superconductive characteristics.This application 18 a division of co-pending application, Serial No.615,814, filed October 15, 1956.

It is known that the electrical resistance of a material decreases withtemperature and that certain materials become superconducting when theyare cooled to a temperature close to absolute zero (0 K.). When amaterial is in a superconductive state, its resistance is equal to zero.It is also known that a conductor having super-conductivecharacteristics may be utilized as a thermal heat switch wherein thenormal or superconduc tive states, respectively, of the conductor passesa heat current easily or acts as a thermal insulator.

When a magnetic field is applied to a superconducting material, thenormal resistance of the material is restored and the material ceases tobe superconductive at a predetermined field strength which is a functionof the temperature and the characteristics of the material. This fieldstrength is known as the critical field of the material. The prior artincludes devices wherein the two states (i.e., the normal andsuperconductive states) of a conductor exhibiting superconductiveproperties are utilized to represent the storage of information or toeffectuate logical control functions. In such structures the conductoris superconductive when the external magnetic field is less than thecritical field and is rendered normal then the magnetic field exceedsthe critical field. In order to maintain such a conductor in asuperconductive state, no external electrical energy need be applied toa circuit incorporating the conductor; but the second state, i.e., thenormal state, is maintainable only through the continuous application ofa magnetic field to the conductor. This magnetic field is generallyproduced by a coil surrounding the conductor, and thus necessitates thecontinual application of electrical current to the coil in order tomaintain the normal state.

The present invention utilizes the phenomenon of induced persistentcurrents induced in a closed current path fabricated fromsuperconductive material. When a closed current path is entirelysuperconductive, a current induced therein will persist since theresistance of the path is zero. A persistent current continues tocirculate in the path without the continuous application thereto ofelectrical energy from an external source. A persistent current iseliminated only by rendering a portion of the path normal for a timesufficient to dissipate the current in the normal resistance introducedin the path. Thus, a closed current path formed of superconductivematerial may exhibit two states which are represented by the presence orabsence of a persistent current therein. Also, the magnitude of acurrent can be stored in the closed path, as where the relativemagnitudes of several currents are representative of information.

The present invention relates to the storage of persistent currents in acontinuous loop of superconductive Corporation, New York,

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material to control the thermal conductivity characteristics of asuperconductive element being used as a heat switch. The invention maybe used to control the flow of heat in a cooling device to establish arefrigeration cycle.

A superconductive material exhibits relatively high and low thermalconductivity in its normal and superconductive states, respectively. Theheat switch includes an additional superconductive element forcontrolling the flow of heat and is disposed adjacent to a portion ofthe superconductive storage loop. The magnetic field created by apersistent current in said loop renders the additional element normal,thus permitting the passage of heat thcrethrough. Thus the presence orabsence of a persistent current in the superconductive loop respectivelyrenders said additional element normal or superconductive to therebyprovide a heat switch capable of controlling thermal conductivity of anelement without requiring the continuous application of electricalenergy to the switch.

Each superconductive storage loop referred to herein as a storage cellmay comprise a single material, or may be fabricated of twosuperconductive materials arranged in series, one having a highercritical field than the other. Means are provided adjacent to eachstorage cell to selectively induce therein, during a Store interval, apersistent current or no current, and further means are provided tosense, during a Read interval, a representation of the presence orabsence of a persistent current in the loop.

During a Store interval, a magnetic field greater than the criticalfield, is applied to a storage cell to render at least a portion thereofnormal. The normal resistance of the normalized portion of the loopdissipates any persistent current previously circulating within theloop. At the termination of the Store interval, the magnetic field isremoved thereby permitting the entire loop to return to itssuperconductive state. If flux from a magnetic field is permitted toencompass a predetermined portion of the storage cell following thetransition from the normal to the superconductive state, a persistentcurrent is induced in the loop when said flux is removed. The persistentcurrent circulates in the storage loop as long as the loop remainsentirely superconductive. However, if following the transition from thenormal to the superconductive state at the termination of the Storeinterval, there is no flux linking said predetermined portion of theloop, a persistent current is not induced therein.

A principal object of the invention is to provide a novel means forcontrolling heat flow in a refrigerator.

Another object is to provide novel means for inducing and storing apersistent current in a closed current path fabricated fromsuperconductive materials whereby the persistent current flowing in saidpath controls the thermal conductivity of a further superconductiveelement.

A further object is to provide a novel control circuit for controllingthe thermal conductivity of a conductor including first and secondsuperconductive materials arranged serially in a closed current path andhaving different critical field values, said conductor being disposedadjacent said first or said second material, means for rendering one ofsaid materials normal by exceeding the critical field thereof, andfurther means for inducing a persistent current in said closed currentpath by selectively controlling the application of a magnetic field tosaid field, thereby controlling the thermal conductivity of saidconductor by the presence or absence of said persistent current.

Another object is to provide a refrigerator having an improved thermalvalve which is controlled by persistent currents flowing in asuperconducting path whereby said valve controls heat exchange in saidrefrigerator.

A further object is to provide a refrigerator having an improved thermalswitch for controlling heat flow therein, said switch employing asuperconducting thermal element which respectively functions as athermal conductor or a thermal insulator when it is in the normal orsuperconductive state, the state of said thermal element beingcontrolled by the presence or absence of a persistent currentcirculating in a conductor adjacent to said element.

Another object is to provide means for utilizing a persistent currentflowing in a superconducting medium to control the extracting of heatfrom a low temperature reservoir by magnetic work producing means.

An object is to provide a novel means for controlling thermal links forselectively providing a heat current path between a paramagnetic saltand a constant temperature reservoir or a low temperature reservoir fromwhich heat is extracted.

It is also an object to provide an improved adiabatic demagnetizationrefrigerator having a paramagnetic salt as a working substance, a firsthigh temperature reservoir, a second low temperature reservoir, a firstthermal switch comprising a thermal link the superconductive state ofwhich is controllable by a persistent current circulating in asuperconducting loop, said first switch disposed between said salt andsaid first reservoir, and a second thermal switch similar to said firstswitch and disposed between said salt and said second reservoir.

A further object is to provide means for controlling a superconductiveheat switch through the use of an induced persistent current circulatingin a superconductive loop.

Other objects of the invention will be pointed out in the followingdescription and claims and illustrated in the accompanying drawings,which disclose, by way of example, the principle of the invention andthe best mode, which has been contemplated, of applying that principle.

In the drawings:

Fig. l is a graph of magnetic field versus temperature fora typicalsuperconductive material;

Fig. 2 is a circuit diagram of a superconductive storage loop forstoring a persistent current;

Fig. 3 illustrates an adiabatic demagnetization refrigerator utilizingstored persistent currents for control purposes; and i Fig. 4 depicts anoperation cycle of the apparatus of Fig. 3.

For each superconductive material a graph of magnetic field versustemperature can be plotted which characterizes the important propertiesof the particular superconductor. The transition curves for lead,niobium, and tantalum are shown in Fig. 1 as curves 10, l1 and 12. Amaterial is said to be in a superconductive state when the relationshipbetween the applied magnetic field and the temperature of the materialis such that the intersection of these values lies in the area beneaththe curve of Fig. 1 corresponding to the material. However, if eitherthe temperature or the magnetic field surrounding the material isincreased whereby the intersection of these values occurs in the areaabove the appropriate curve, the material is said to be in the normalstate.

With respect to Fig. 1, consider that the superconductor is lead, forexample, andis cooled to temperature T. As long as the magnetic fieldapplied to the conductor is less than the value HAT), the conductor willexist in a superconductive state. If the magnetic field is now increasedabove the value H (T), the conductor is transformed to the normalconductive state. The field strength H at which the transition from thesuperconductive to the normal state occurs is called the critical field.Hence, it is seen that when the temperature of a superconductor ismaintained at a constant value, the increasing and decreasing of themagnetic field controls the resistance of the'conductor by causing theproperties thereof to shift back and forth between its superconductingnormal states 4- respectively. Fig. 1 also indicates that in order tocontrol the conductive state of a superconductor by controlling themagnetic field, the temperature of the con ductor must be maintained ata value below the transition temperature corresponding to zero magneticfield.

The magnetic field may be an externally applied field or may be producedby the current flowing through the superconductive element, or may be acombination of both of ,these fields. The critical magnetic field H (T)limits the current which can be passed through the superconductorwithout destroying the superconductive state. The magnetic field at thesurface of a cylindrical conductor, due to the current flowingtherethrough, is equal to 21 lOr, where r is the radius of the wire incentimeters and I is the critical current corresponding to the criticalfield H (T). v With respect to curves l0 and 11 (Fig. 1) note that whenthe system is operating at approximately 4 K., for example, the criticalfield I- I (T), sufficient to render a lead conductor normal (curve 10),is insufiicient to render a niobium conductor normal. From the plot ofFig. 1, it is obvious that the critical field for niobium at 4 K. ismany times larger than the critical field for lead. Thus it is clearthat several superconducting elements being operated in the samevicinity are responsive to different field strengths so that the stateof one superconductive element can be controlled by the magnetic fieldin the vicinity Without affecting the state of other nearby conductorshaving a higher critical field. Where a superconductive material such aslead, for example, is utilized in the vicinity of another material, suchas, niobium, and the respective materialshave radically differentcritical fields, the material having the lower critical field isreferred to as a soft superconductor, whereas the material having thegreater critical field is referred to as a hard superconductor.

While Fig. 1 only illustrates the transition curves for lead, niobium,and tantalum, a similar curve can be plotted for any superconductivematerial. The nature of tin, for example, is such that a plot of itstransition curve would appear beneath curve 10 of Fig. 1. When it isdesired to obtain a material having a critical field intermediate, afirst material such as tin, for example, and a second material such aslead,for e xample, a homogeneous alloy of the two materials may often beused in order to provide a material having the desired intermediatecritical field value.

Under certain conditions it is desirable that a superconductivernaterial, when rendered normal, exhibit a high normal resistance. Ahigher resistance can be obtained by plating a superconductive materialsuch as lead, for example, onto a graphited plasticbase The increasedresistivity appears only when the material is normalized, and isshortedwhen it becomes superconductive.

It is known that whenamagnetic flux links a loop of material at the timethat said material passes from its normal to its superconductive stateand the flux is later removed, a current is induced in the loop whichthereafter persists and continues to circulate therein. Such a currentis known as a persistent current. A persistent current will circulate ina superconducting loop until a portion of the loop is rendered normalwhereby the normalresistance of the normalized portion is introducedinto the loop. A persistent current is dissipated in the normalresistance referred to above.

5 which the presence or absence of a persistent current in asuperconducting loop is detected.

Further information concerning superconductive materials, theories ofsuperconductivity and a synopsis of the experiments performed to date onsuperconductive materials may be found in the following: D. Schoenberg,Superconductivity, second edition, The Syndics of the CambridgeUniversity Press, London, England (1952); M. Von Laue, Theory ofSuperconductivity, Academic Press Inc., New York, NY. (1952); and D. A.Buck, The Cryotron-A Superconductive Computer Component, Proceedings ofthe I.R.E., vol. 44, No. 4, pp. 482-493, April 1956. These referencesalso include further references to literature relating to methods ofobtaining operating temperatures near K. by apparatus utilizing liquidhelium.

Referring more particularly to Fig. 2, a novel circuit for inducing andstoring a persistent current in a superconducting loop is illustrated.All of the components shown within the dashed rectangle 16 of Fig. 2 aremaintained at a temperature corresponding to temperature T, for example,of Fig. 1. The temperature at which these elements must be maintained isdependent upon the superconductive materials utilized, and may be in therange of 2 K. to 5 K.

The superconductive storage loop of Fig. 2 comprises a conductor 18 andan inductance 19 connected in parallel between terminals 20 and 21.Conductor 18 is fabricated from a superconductive material having arelatively smaller critical field than the critical field associatedwith the superconductive material from which inductance 19 isfabricated. If preferred, conductor 18 and inductance 19 may befabricated from materials having similar critical fields, in which casethey must be physically separated so that a magnetic field applied toone does not affect the other. As explained hereinbelow, the inductance19 always remains superconductive, whereas conductor 18 will be shiftedbetween its normal and its superconductive state. An inductance 22surrounds conductor 18 and may be fabricated from a superconductivematerial having a relatively high critical field as compared withconductor 18. However, it is not essential to the invention that thisinductance be superconductive. Inductance 22 is connected betweenterminals 23 and 2-1. Inductance 22, like inductance 19, always remainsin the superconductive state.

A voltage source 24 is connected to supply a current between terminal 21and resistors 25 and 26 which are respectively connected between thesource 24 and switches 27 and 28.

A read gate and amplifier circuit 30 is provided which receives thevoltage signals developed between terminals 20 and 21. The output of theread circuit is connected to terminal 31. The read circuit 30 is gatedso as to amplify any voltage signal appearing at terminals 20 and 21during a Read interval. Where the device of Fig. 2 is used to control athermal switch ina refrigerator, the read gate and amplifier circuit 30may be eliminated, or alternatively in a complex arrangement, may beused to indicate the status of the thermal switch.

Briefly, the switches 27 and 28 of Fig. 2 are actuated in the propersequence to induce a persistent current in the loop comprising conductor18 and inductance 19. After a persistent current is induced in the loop,the current continues to circulate therein without the application ofelectrical energy from an external source. The persistent current willcontinue to circulate in the loop indefinitely or until some portion ofthe loop, such as conductor 18, is rendered normal for a period of timesufficient to permit the current to be dissipated in the normalresistance of the conductor- The conductive state of conductor 18 iscontrolled by the magnetic field surrounding inductance 22. For examplewhen a current is flowing through inductance 22, having a valuesufficient to create a field greater than the critical field ofconductor 18, the latter is rendered normal. Upon the removal of thisfield, conductor 18 reverts to the superconductive state. The currentapplied to the inductances 22 and 19 of Fig. 2 must be limited so thatthe fields created about the inductances do not render the inductancesthemselves normal, but rather permit the inductances to always remain inthe superconductive state.

The material comprising conductor 18 may be selected to have a lowercritical field than the material comprising inductances 19 and 22. Thusconductor 18 may be fabricated, for example, of lead or tantalum and theremaining conductors within rectangle 16 of Fig. 2 may be composed ofniobium. However, there are many materials such as vanadium, aluminum,tin, titanium, and alloys thereof, to name only a few, which exhibitsuperconductive properties and may be used for the superconductiveelements of Fig. 2.

Consider, for example, that all the conductors within the rectangle 16of Fig. 2 are in their superconductive states. If switch 27 is closed, acurrent I supplied by voltage source 24 is applied through the switch toterminal 23, through inductance 22 to terminal 21 and returns to thegenerator 24. Current I applied to inductance 22 must be sufficient toproduce a magnetic field within the inductance having a magnitudegreater than H (T) so as to destroy the superconductive state ofconductor 18. Hence, whenever switch 27 is closed, conductor 18 is madenormal.

If, while switch 27 remains closed, switch 28 is closed, current I isapplied via terminal 20 to the parallel combination of conductor 18 andinductance 19. This current returns via terminal 21 to voltage source24. Current I flows entirely through the superconducting inductance 19since the inductance has no resistance, where as conductor 18 is nowexhibiting its normal resistance due to the magnetic field created byinductance 22. Several settling times must transpire before the currentis flowing entirely through the inductance 19.

Switch 27 is now opened causing the field within inductance 22 tocollapse, thus rendering conductor 18 normal. The current I continues toflow through inductance 19 even though conductor 18 is nowsuperconductive. Thereafter, switch 28 is opened and the current ininductance 19 attempts to decrease. The energy stored in the inductanceforces the current flowing therein to flow through conductor 18. Sinceconductor 18 is now superconducting, the current flowing in inductance19 begins to circulate as a persistent current in the storage loopcomprising inductance 19 and conductor 18. The persistent currentinduced in the loop is proportional to the magnitude of the currentflowing through inductance 19 and in most. cases is very nearly equal toit.

The induced persistent current circulates in the storage loop withoutthe further application of current thereto. This persistent current willcontinue to circulate for several years without any appreciable changein magnitude, providing the superconductive loop is maintained at theproper temperature and is not subjected to an external magnetic fieldgreater than the cn'tcal field of any of the components of the loop.

The persistent current circulating within the storage loop can bedestroyed by closing switch 27 for several settling times. The closureof switch 27 establishes a magnetic field within inductance 22 whichdestroys the superconductive state of conductor 18. The persistentsupercurrent is then dissipated by the normal resistance of conductor18.

In certain applications, as Where the storage cell is utilized as amemory device, for example, it is desirable to sense the existence of apersistent current in the storage cell of Fig. 2. During a Readoperation, switch 28 remains open. The closure of switch 27 causes acurrent to flow through inductance 22, thereby applying a magnetic fieldto conductor 18. The superconductivity of conductor 18 is destroyed bythe magnetic field, and

the conductor exhibits its normal resistance. The persistent currentcirculating through inductance 19 and conductor 18 decreases when itencounters the normal resistance 'R of conductor 18. The current throughconductor 18 produces a voltage signal between terminals 20 and 21. Thissignal is gated and amplified by the read gate and amplifier circuit ofFig. 2 and appears at output terminal 31. The read-out signal appearingat terminal 31 can be applied to any suitable circuitry, such as theread-in circuits of a digital computer.

It is to be appreciated that the switches 27 and 28 of Fig. 2 are merelysymbolic, and normally comprise electronic or superconductive switchingmeans.

One of the advantages of the invention is that the storage of apersistent current may be of a permanent nature. In an apparatus whichutilizes the invention, for example, the stored current is not loss whenthe power supplies fail. Further, the persistent current type storagecell is easily constructed and economically operated. The circuit ofFig. 2 may also be used in an analog type computer and in other storageand control applications, since the persistent current induced in thestorage loop is proportional to the field about inductance 19. That is,the circuit may be used to store the magnitude of a current.

It should be stressed that once information is stored as a persistentcurrent in a superconductive loop, the information is continuouslystored as long as the entire loop remains superconductive. Thus, inorder to destroy the stored information, at least a portion of thesuperconductive loop must be rendered non-superconductive for severalsettling times. Such a loop can be rendered non-superconductive byraising the temperature above the transition temperature, or by applyingthereto a magnetic field greater than the critical field.

Referring more particularly to Fig. 3 an adiabatic demagnetizationrefrigerator incorporating the invention for controlling a thermal heatswitch is illustrated. It is known that a conductor havingsuperconductive characteristics may be utilized as a thermal heat switchwherein the normal or superconductive state of the conductorrespectively passes a heat current easily or acts as a thermalinsulator. Thus by placing a superconducting conductor within theinductance 19 of Fig. 2, the novel circuit of Fig. 2 may be utilized tocontrol the thermal properties of said conductor, thereby providing athermal heat switch. The opening or closing of the heat switch isrespectively determined by whether or not a magnetic field greater orless than the critical field is applied to the conductor. The selectiveapplication of such a magnetic field may be efiected by controlling apersistent current circulating in a superconducting storage cell of thetype described hereinbefore.

Superconductive thermal switches are found in the prior are wherein themagnetic field used to control the superconductive element is providedby an electromagnet surrounding said element. In order to maintain theelement in the thermal conducting state, a current of several amperesmust be continuously supplied to the electromagnet. Accordingly, thelarge power requirements of the electromagnet demand that the powersupplies be capable of delivering large currents for time intervals upto 100 seconds. These requirements dictate substantial and more costlypower equipment. Further considerable power is dissipated in theelectromagnet.

In the novel thermal switch described herein the necessity for asustained current to provide the required magnetic field is eliminated.A superconducting loop including an inductance is provided. Theinductance is disposed adjacent the superconducting thermal element.Current pulses are utilized to induce a persistent current in thesuperconducting loop. Once established, the persistent currentcirculates in the loop without the further application of electricalenergy thereto. The persistent current flowing through the inductancecreates a magnetic field whi h re der e lem n t rmal c nd c i e Si e thecurrent pulses which induce-the persistent current are of less thanfifty microseconds duration, the equipment necessary to produce them isless costly than the heavyduty power supplies heretofore required. Also,there is no power dissipation in the loop, thus increasing theefficiency of the entire system.

The low temperature components of the demagnetization refrigerator areenclosed within the thermal insulating vacuum chamber 200 of Fig. 3. Thecontainer 201 of Fig. 3 serves as a constant high temperature reservoirand is generally filled with liquid helium which has a temperature ofapproximately 1K. However, other substances may be used in container 201in order to provide a different reference temperature. Container 202 isfilled with a paramagnetic salt such as iron-ammonium alum or chromiumpotassium alum, and is referred to herein as salt pill P Theparamagnetic salt is used to perform the work accomplished in obtaininga temperature lower than the reference temperature of the reservoir 201.A paramagnetic salt is also used as .a low temperature reservoir whichis housed in container 203. The low temperature reservoir is referred toherein as salt pill P The constant-temperature reservoir 201 is coupledto the paramagnetic salt pill P by thermal switch 204 and salt pill P iscoupled to the working substance P by the thermal switch 205. Briefly,the paramagnetic salt pill P which serves as the working substance ismagnetically controlled to absorb heat flowing from the low temperaturereservoir P and the constant temperature reservoir 201, in turn, absorbsheat from the working substance P Heat switch 204 includes a thermallink 208 fabricated from a superconducting material. The link 208 may,for example, be pure lead which exhibits good thermal conductivity inthe non-superconducting state. Thermal link 208 is bonded to members 209and 210 which respectively provide a good thermal conductive path fromthe link to container 201 and the working substance P A thermalinsulating member 211 supports the thermal conducting members 2&39 and210.

The superconductive or normal state of thermal link 208 is controlled byinductance 212 which is connected in parallel with a superconductingconductor 213. Conductor 213 may be fabricated from an alloy of tin andlead, for example, in order to reduce the critical field required tonormalize the conductor. A first juncture of conductor 213 andinductance 212 is connected to terminal 214, and the second juncture ofthese members is connected to terminal 215. The conductor 213 issurrounded by the superconducting inductance 216 which is connectedbetween terminals 215 and 217. A brief comparison of thermal switch 204with the circuit of Fig. 2 indicates that terminals 214, 217 and 215(Fig. 3) respectively correspond to terminals 20, 23 and 21 of Fig. 2.

A thermal link 220 of heat switch 205 is bonded to members 221 and 222which respectively serve as thermal conductors between link 220, theworking substance P; and reservoir P A superconducting inductance 223surrounds thermal link 220 and is connected in parallel with asuperconductive conductor 224. The two junctures of the parallelcombination of conductor 224 and inductance 223, are respectivelyconnected to terminals 225 and 226. A further superconducting inductance227 surrounds conductor 224 and is connected between terminals 228 and226. The thermal conducting members 221 and 222 are supported by athermal insulating member 230. It is to be noted that the constructionof thermal switch 205 is identical with switch 204.

As stated hereinabove, thermal switch 204, for example, serves tocontrol the flow of heat currents between working substance P andconstant temperature reservoir 2131. When the thermal link 208 isrendered normal, the thermal resistance of the link is relatively low sothat heat currents are permitted to pass therethrough. How ever, whenthe link 208 is in the superconductive. state. it

acts essentially as a thermal insulator. When a persistent current iscirculating in the superconductive loop comprising inductance 212 andconductor 213, a magnetic field is created around inductance 212 whichis applied to thermal link 208. This field is greater than the criticalfield of link 208 and thus renders it normal so that heat currents maypass therethrough. On the other hand, if a persistent current is notcirculating within the parallel combination of inductance 212 andconductor 213, the thermal link remains in its superconductive state soas to act as a thermal insulator. The manner in which a persistentcurrent is induced in the parallel combination of inductance 212 andconductor 213, is described hereinabove with respect to Fig. 2. Thus itis seen that a persistent current circulating in a superconductingclosed current path may be used to control a thermal link fabricated ofsuperconductive material, thereby providing the functions of a thermalswitch.

The work performed within the demagnetization refrigerator iseffectuated by magnetizing and demagnetizing the paramagnetic salt Pconstituting the working substance. The magnetic properties of theworking substance are controlled by electromagnet 234 which is arrangedexternal to vacuum chamber 200. The winding of the electromagnet isrespectively connected between terminals 235 and 236.

Briefly, the cycle of operation of the adiabatic demagnetizationrefrigerator is as follows. Firstly, the paramagnetic salt P ismagnetized by applying a current I to terminals 235 and 236. Secondly, apersistent current is established in' thermal switch 204 so that theparamagnetic salt 202 is thermally connected to the constant temperaturebath 201. The heat of magnetization created within the paramagnetic saltP is then conducted to the constant temperature reservoir 201 throughthe normalized thermal link 208. Thirdly, the persistent currentcirculating in thermal switch 204 is destroyed so that thermal link 208becomes superconductive thereby thermally insulating paramagnetic salt Ffrom the constant temperature reservoir 201. Fourthly, the current I isdecreased so that the paramagnetic salt 202 is demagnetized. Fifthly, apersistent current is established in thermal switch 205 so as tonormalize the thermal link 220. The link 220 then provides a thermalpath from the reservoir 203 to the paramagnetic salt 202. Upondemagnetization, the salt pill P cools to about 01 K. When thermal link220 becomes thermally conductive, the temperature of salt pills P and Pequalize. Lastly, after the temperatures of the reservoir P and theparamagnetic salt P have equalized, the persistent current circulatingin thermal switch 205 is destroyed. The removal of the magnetic fieldfrom thermal link 202 renders the link superconductive and thusthermally insulates the reservoir 203 from the salt 202. The cycle isnow repeated to continue the extraction of heat from the reservoir 203.Note that the structure is arranged so that all heat flow is upwards,i.e., from P to P and from P to reservoir 201.

A detailed description of the operation of a demagnetizationrefrigerator similar to that of Fig. 3 is contained in Heer, Barnes andDaunts article The Design and Operation of a Magnetic Refrigerator forMaintaining Temperatures Below 1 K., Review of Scientific Instruments,vol. 25 No. 11, pages 1088-1098, November 1954. i

The apparatus of Fig. 3 is a single stage refrigerator. In order toobtain even lower temperatures a second stage may be added below pillPso that P would serve as the high temperature reservoir of a secondstage.

The operation of the thermal switch 204 and 205, .in order to providethe cycle of operation of the demagnetization refrigerator, isillustrated by the diagram of .Fig. 4. Fig.4 depicts the current pulsesapplied to the thermal switches204 and 205, the waveform of the currentI applied to magnet 234 and the temperature gradient of salt pills P andP Referring to Fig. 4, the current I is applied to magnet 234 during theinterval t During this interval, the paramagnetic salt pill P ismagnetized causing the temperature thereof to increase above 1 K.Beginning at time a current pulse 1 is applied to terminal 217 causingconductor 213 to be rendered normal. Simultaneously therewith, a currentpulse I is applied to terminal 214 which establishes'current flowthrough inductance 212. As indicated in Fig. 4, current I remains onafter the cessation of 1 thereby inducing a persistent current in theloop comprising inductance 212 and conductor 213, in the mannerdescribed hereinbefore with respect to Fig. 2. Accordingly, the thermallink 208 is rendered normal by the field produced by the current flowingthrough inductance 212. The normalization of link 208 creates a thermalconductive path from salt pill P to constant temperature reservoir 201.As indicated in Fig. 4 by the temperature T the temperature of pill Pequalizes to the temperature of the constant temperature reservoir 201.During interval t the current I remains constant and thus pill P remainsmagnetized.

At the termination of interval 2 a current pulse 1 is applied toterminal 217 (Fig. 3) which renders conductor 213 normal. Thenormalization of conductor 213 destroys the persistent currentcirculating in thermal switch 204. Also, the current I begins todecrease toward zero during interval t The decreasing current throughmagnet 234 (Fig. 3) demagnetizes salt pill P which then cools to atemperature slightly below approximately 0.1 K.

At the commencement of time interval t current pulses I and I arerespectively applied to terminals 228 and 225 (Fig. 3) thereby inducinga persistent current in the superconducting loop comprising inductance223 and conductor 224, in the manner described above. The persistentcurrent in thermal switch 205 renders thermal link 220 normal so that athermal conductive path is established between salt pills P and P Hence,during interval t the temperatures of pills P and P equalize therebydecreasing the temperature of pill P At the termination of interval t asecond I current pulse is applied to terminal 225 which destroys thepersistent current circulating in thermal switch 205. The destruction ofthis current permits thermal link 220 to become superconducting so as tothermally insulate salt pills P and P This completes one cycle of theadiabatic demagnetization refrigerator of Fig. 3. During the interval twork is not performed in the refrigerator. The duration of interval 1 isdependent upon the frequency with which the cycle of the refrigeratormust be repeated in order to maintain salt pill P at approximately O.1K.

It is indicated in Fig. 3, that the temperature T of salt pill Pgradually rises from the beginning of the cycle through the end ofinterval t During interval 22;, the temperature of salt pill P isdecreased since it is cooled to the temperature of pill P At theconclusion of interval t the temperature of salt pill P increases untilthe occurrence of another interval similar to 1 The temperature riseduring intervals t through t of salt pill P and also the frequency withwhich the cycle of the refrigerator must be repeated, is dependent uponthe heat losses in the refrigerator of Fig. 3.

When the refrigerator of Fig. 3 is utilized to cool a substance toapproximately 01 K., the substance is placed in thermal contact withsalt pill P An appropriate aperture or connection means in order toattach the substance to be cooled to P must be provided, and such meansis not illustrated in Fig. 3 since any wellknown structure may beutilized.

While there have been shown and described and pointed out thefundamental novel features of the inventron as applied to a preferredembodiment, it will be 3 ll understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in theart, without departing from the spirit of the invention. The invention,therefore, is to be limited only as indicated by the scope of thefollowing claims.

What is claimed is:

1. An adiabatic demagnetization refrigerator comprising the combinationof; a paramagnetic salt pill; means for alternately magnetizing anddemagnetizing said salt pill, first means having a constant temperature;a first thermal valve coupling said first means and said pill forequalizing the temperatures thereof after said pill is magnetized;second means operated at a lower temperature than said first means; asecond thermal valve coupling said pill and said second means forequalizing the temperatures thereof after said pill is demagnetized tothereby decrease the temperature of said second means; said first andsecond valves each comprising a superconductive element reacting as athermal insulator and a thermal conductor when respectively in thesuperconductive and normal states, a superconductive loop magneticallycoupled to said element for controlling the superconductive state ofsaid element, means for inducing a current in said loop which persiststherein without the further application of electrical energy to saidloop to thereby render said element a thermal conductor, and meanscoupled to said loop for effecting the dissipation of a persistentcurrent in said loop whereby said element is rendered a thermalinsulator.

2. A magnetic refrigerator including the combination of, constanttemperature means, a second temperature means operated at a lowertemperature than said constant temperature means, a material havingparamagnetic properties for producing a decrease in temperature, a firstsuperconducting link thermally insulating said constant temperaturemeans and said material, a second superconducting link thermallyinsulating said material and said second temperature means, first andsecond superconductive means for storing persistent currents andrespectively coupled to said first and second links, first means forestablishing a persistent current in said first superconductive meansfor a predetermined time interval to render said first link thermallyconductive thereby equalizing the temperatures of said constanttemperature means and said material, and second means for establishing apersistent current in said second superconductive means during apredetermined interval to render said second link thermally conductivethereby equalizing the temperatures of said material and said secondtemperature means.

3. An adiabatic demagnetization refrigerator including the combinationof, a constant temperature reservoir, means including a paramagneticsalt for producing a temperature differential, means for alternatelymagnetizing and demagnetizing said salt, a first superconductive thermalswitch controlled by a persistent current circulating in a closedsuperconducting path for controlling the flow of heat currents betweensaid constant temperature reservoir and said salt, a low temperaturereservoir, and a second superconductive thermal switch for controllingthe fiow of heat currents between said low temperature reservoir andsaid salt, whereby said first switch effects equalization of thetemperatures of said constant temperature reservoir and said salt afterthe latter is magnetized and said second switch effects equalization ofthe temperatures of said salt and said low temperature reservoir aftersaid salt is demagnetized.

4. A magnetic refrigerator having a predetermined cycle of operationincluding the combination of, a first means having a temperature in thesuperconductive region, a second means normally subsisting at a lowertemperature than said first means, third means for producing atemperature drop, a first thermal valve coupling said first means andsaid third means for establishing the lat-. ter at the temperature ofthe former, said valve including means for storing a persistent currentto control the thermal conductivity of said valve, and a second thermalvalve for establishing a thermal connection between said second meansand said third means when said third means subsides to its lowesttemperature level, whereby said second means is cooled to a temperaturebelow that of said first means during each cycle of operation.

5. A magnetic refrigerator for producing a temperature lower than areference temperature including the combination of, means for achievinga predetermined temperature decrease from said reference temperature, areservoir, and a thermal valve coupling said reservoir and said meansfor establishing said reservoir at the lowest temperature excursion ofsaid means, said valve including a closed superconducting path forstoring a persistent current to control the thermal conductivity of saidvalve. 1

6. A magnetic refrigerator for producing a temperature lower than areference temperature including the combination of, means for achievinga predetermined temperature decrease from said reference temperature, areservoir, and a thermal valve coupling said reservoir and said meansfor establishing the former at the lowest temperature excursion of thelatter, said valve including a first superconductive element capable ofacting as a thermal conductor and a thermal insulator, and a secondsuperconductive element having two operative states and coupledto saidfirst element for alternately rendering the latter a thermal conductorand a thermal insulator when said second element is respectively in itsfirst and second states.

7. A magnetic refrigerator including the combination of, a constanttemperature means, means including a paramagnetic material for achievinga temperature decrease, a first superconductive thermal switch renderedoperative by a persistent current circulating in a closedsuperconducting path for establishing said paramagnetic material at saidconstant temperature, means maintainable at a lower temperature thansaid constant temperature, and a second superconductive thermal switchfor establishing said last-named means at the lowest temperatureexcursion of said paramagnetic material.

8. A magnetic refrigerator including the combination of, constanttemperature means, a second temperature means operated at a lowertemperature than said constant temperature means, paramagnetic means forproducing a temperature reduction, a first thermal valve connectedbetween said constant temperature means and said paramagnetic means, asecond thermal valve connected between said paramagnetic means and saidsecond temperature means, said first and second thermal valves each in:cluding a superconductive element capable of assuming thermal conductingand thermal insulating states, a first superconductive means forcontrolling the thermal state of said superconductive element of saidfirst valve, and second superconductive means for controlling thethermal state of said superconductive element of said second valve,whereby said first valve effects the equalization of the temperatures ofsaid constant temperature means and said paramagnetic means prior to thelatter eifecting a temperature decrease and said second valve effects anequalization of the temperatures of said paramagnetic means and saidsecond temperature means when the former has achieved its lowesttemperature excursion.

References Cited in the file of this patent The Design and Operation ofa Magnetic Refrigerator for Maintaining Temperature below 1 degree K. inthe Review of Scientific Instruments, volume 25, Number 11, pages1088-1098, November 1954.

Magnetic Refrigerator, in Mechanical Engineering, pages 1088 and 1089,December 1955.

